softpipe: fix off-by-one error in setup_fragcoord_coeff()
softpipe: fix off-by-one error in setup_fragcoord_coeff()

If we invert Y, need to subtract one from the surface height.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=26795
for softpipe.

NOTE: This is a candidate for the 7.9 and 7.10 branches.

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